The law also calls for the closure of non-governmental organizations receiving American funding if their activities are classified as political - a broad definition many fear could be used to close any NGO that offends the Kremlin.

The law takes effect Jan. 1, the Kremlin said. Children's rights ombudsman Pavel Astakhov said 52 children who were in the pipeline for U.S. adoption would remain in Russia.

Jan Wondra, Vice Chair of the Families for Russian and Ukrainian Adoption says the U.S. State Department and the American Embassy in Moscow will be working overtime this weekend to help the families who are on the verge of completing their adoption process.

"It's like an orphan airlift out of Russia of the children whose families have their decrees," Wondra said. "They have their decrees but then they have to have the paperwork to leave Russia."

Wondra said the total number of kids going through different parts of the adoption process in Russia ranges between 500 to 1,000. It takes months and three trips to adopt a child in Russia.

"State [Department] is going to advocate on behalf of them which is also by the way an unusual step for the State Department . It works on the long term in the wider plain, on behalf of all children not specific cases. But in this case being aware of how tenuous this situation is, a human tragedy it represents, they're looking at all the cases, trying to determine what stage they're in, so they can advocate in mass for the people who, the children who are in process."

The ban is in response to a measure signed into law by President Barack Obama this month that calls for sanctions against Russians assessed to be human rights violators.

That stems from the case of Sergei Magnitsky, a Russian lawyer who was arrested after accusing officials of a $230 million tax fraud. He was repeatedly denied medical treatment and died in jail in 2009. Russian rights groups claimed he was severely beaten.

A prison doctor who was the only official charged in the case was acquitted by a Moscow court on Friday. Although there was no demonstrable connection to Putin's signing the law a few hours later, the timing underlines what critics say is Russia's refusal to responsibly pursue the case.

The adoption ban has angered both Americans and Russians who argue it victimizes children to make a political point, cutting off a route out of frequently dismal orphanages for thousands.

"The king is Herod," popular writer Oleg Shargunov said on his Twitter account, referring to the Roman-appointed king of Judea at the time of Jesus Christ's birth, who the Bible says ordered the massacre of Jewish children to avoid being supplanted by a prophesied newborn king of the Jews.

A painting depicting the massacre and captioned "an appropriate response to the Magnitsky act" spread widely on the Internet. The phrase echoed Putin's characterization of the ban while it was under consideration.

U.S. State Department spokesman Patrick Ventrell expressed regret over Putin's signing the law and urged Russia to "allow those children who have already met and bonded with their future parents to finish the necessary legal procedures so that they can join their families."

Wondra said there are not enough families in Russia to adopt over 700,000 orphans and abandoned children. Wondra also said that Russian families like to adopt babies, a 3-year-old is already too old. They don't usually adopt children with special needs.

Vladimir Lukin, head of the Russian Human Rights Commission and a former ambassador to Washington, said he would challenge the law in the Constitutional Court.

The U.S. law galvanized Russian resentment of the United States, which Putin has claimed funded and encouraged the wave of massive anti-government protests that arose last winter.

The Parliament initially considered a relatively similar retaliatory measure, but amendments have expanded it far beyond a tit-for-tat response.

UNICEF estimates that there are about 740,000 children not in parental custody in Russia while about 18,000 Russians are on the waiting list to adopt a child. The U.S. is the biggest destination for adopted Russian children - more than 60,000 of them have been taken in by Americans over the past two decades.

Russians historically have been less enthusiastic about adopting children than most Western cultures. Putin, along with signing the adoption ban, on Friday issued an order for the government to develop a program to provide more support for adopted children.

Lev Ponomarev, one of Russia's most prominent human rights activists, hinted at that reluctance when he said Parliament members who voted for the bill should take custody of the children who were about to be adopted.

"The moral responsibility lies on them," he told Interfax. "But I don't think that even one child will be taken to be brought up by deputies of the Duma."

Many Russians have been distressed for years by reports of Russian children dying or suffering abuse at the hands of their American adoptive parents. The new Russian law was dubbed the "Dima Yakovlev Bill" after a toddler who died in 2008 when his American adoptive father left him in a car in broiling heat for hours.

In that case, the father was found not guilty of involuntary manslaughter and Russia has complained of acquittals or light sentences in other such cases.

The Investigative Committee, Russia's top investigative body, on Friday complained that its attempts to have the acquittals overturned or reconsidered had been ignored by the United States. Under U.S. law, acquittals are final except in rare cases.

Russians also bristled at how the widespread adoptions appeared to show them as hardhearted or too poor to take care of orphans. Astakhov, the children's ombudsman, charged that well-heeled Americans often got priority over Russians who wanted to adopt.

A few lawmakers even claimed that some Russian children were adopted by Americans only to be used for organ transplants or become sex toys or cannon fodder for the U.S. Army. A spokesman for Russia's dominant Orthodox Church said that children adopted by foreigners and raised outside the church will not enter God's kingdom.